The present invention related to ultraviolet laser light sources, e.g., semiconductor manufacturing lithography laser light sources, elongated thin beam thin film panel treatment (amorphous silicon annealing for form elongated polysilicon crystals) and extreme ultraviolet plasma formation drive lasers, and the like, and particularly to pulse energy and dose control for such systems.
An exemplary energy controller aims to keep the measured laser system output pulse energy of each pulse or pulse energy averaged over a selected number of pulses, e.g., within a given burst, constant at the setpoint specified (e.g., at 10 mJ), though often the pulse energy can vary from the selected set point. In the case of energy control system such as have appeared in applicants' assignee's laser systems in the past, such as XLA series laser systems sold by Cymer, Inc. such energy control may have been achieved by calculating a voltage to command to the pulse power sub system that is computed from the energy errors from previous pulses (e.g., measured shutter energy at the laser system output shutter minus the energy setpoint). Such a system is illustrated in FIG. 1 and may contain a laser system 20 having a laser 22 which outputs a laser beam of pulses 24 each of which may have a measured energy, Emeasured and a controller 30, which may include a summer 32 for subtracting an energy setpoint, Esetpoint, from Emeasured to provide and Energy Error, Eerror to and energy controller servo 36, which may perform an algorithm to produce a voltage command signal Vcommand to the laser 22, more specifically to a timing and energy controller for the laser or directly to a discharge voltage system, such as to the resonant charger. Alternatively, the servo 36 may be part of the timing and energy controller or the laser system and provide the Vcommand signal to the solid state switched pulsed poser system (SSPPM) as are known in the art to control the voltage delivered between the electrodes in the laser for the next discharge produced laser pulse in the beam of laser pulses produced by the laser system 20. A trigger signal may be produced, e.g., either the laser timing and energy controller or some external source, e.g., a light source triggering controller on, e.g., an application system using the light, such as an integrated circuit photolithography system, e.g., a scanner, or a thin film transistor crystallization system or a laser produced plasma extreme ultraviolet light source plasma generation system. Discrete trigger signals may be received by the laser or desired time of firing signals may be received, which together with other prior trigger signals or desired time of firing signals may be used to define a trigger interval. Such trigger interval(s) may, however, vary from time to time or over time according to the timing of the trigger signals produced by the laser timing and energy control system or the external source.
The laser system 20 may be a discrete laser system (i.e., pulsed), which means pulses may be requested at varying intervals in various ways as just noted. The interval between the laser pulses is referred to herein as the “trigger interval”. The laser efficiency can change significantly as the trigger interval is varied. FIG. 3 shows an example of the resulting energy when such a laser system 20 is pulsed in constant voltage (i.e. the energy controller disabled so that no effort is made to modify discharge voltage to maintain energy constant, or as nearly so as possible with current controllers) for 15 patterns 40 consisting of 50 bursts, with a 30 second pause between each pattern 40. The x-axis is in seconds. The duty cycle was increased from 5% on the first pattern 40 to 40% on the last pattern 40 in equal increments of 2.5% each time successively.
A slowly varying duty cycle based change in laser efficiency has been observed, as illustrated, and denoted by the offset line 42. This effect is slow enough (order of minutes) that an energy controller servo, e.g., servo 36 in FIG. 1 can effectively flatten this to a desired energy offset line, i.e., maintain a desired pulse energy setpoint.
Applicants provide an improved method and apparatus for maintaining this desired energy setpoint especially at the initiation of a burst.